1. Field of the Invention
The invention relates to a photosensitive elastomer composition having enhanced exposure latitude by using actinic radiation absorbers that are distributed evenly throughout the photosensitive elastomer composition. The invention also relates to a printing element formed from the photosensitive elastomer composition and a method of forming the printing element.
2. Description of the Prior Art
Photosensitive printing elements used for flexographic printing crosslink or "harden" and become insoluble upon exposure to actinic radiation or light. Generally, a template, such as a photographic negative or transparency is put onto an unexposed plate containing a layer of photosensitive material. The plate is then exposed to actinic radiation, such as UV or black light, close to the plate. The actinic radiation enters the photosensitive material through the transparent regions as divergent beams of light and irradiates a continually diverging area in the material underneath the clear areas of the transparency. The exposed material crosslinks and becomes insoluble to solvents used during the development of the image on the plate. The crosslinked material has its greatest width at the bottom of the photosensitive layer, thus forming a frustum, while the top surface is the same size as the template. Unexposed areas remain uncrosslinked and are washed away with a suitable solvent, thus producing a final plate as a relief, an intaglio or a combination of both. Then the plate is dried. The washed printing plate can be further treated to remove surface tackiness and exposed to additional actinic radiation to ensure complete photopolymerization and crosslinking. After these steps, the plate is ready to be mounted on a cylinder and used for printing.
Good flexographic printing plate physical properties include softness, flexibility, resilience and durability. These properties affect print quality. Plates should print with a "kiss" impression under very low impression pressure. That is, they pick up the ink and accurately transfer the image without distortion. Hard plates pick up less ink, transfer the image unevenly and require greater impression pressure. A soft plate is thus preferable to a hard plate.
A problem exists with flexographic plates regarding latitude in the time of exposure to actinic radiation that is addressed by the present invention. The finest relief or above surface detail is best produced by longer exposure times, while the intaglio or below surface detail is favored at shorter times. Flexographic plates are therefore exposed in a variety of ways to ensure proper image development. Unfortunately, it is sometimes necessary to mask off portions of the plate and give them different exposure times, which is both tedious and undesirable. Therefore, one definition of the exposure latitude of a printing plate is its ability to reproduce different types of detail with a single exposure to actinic radiation. For example, a test may measure the finest dots in relief that hold (that is, do not wash away) at a given UV exposure. Loss of such fine relief dots occurs when the dots are underexposed and have little or no shoulder. If not removed during washing due to underexposure, undercutting may occur to yield a pedestal-like structure similar to a mushroom which is also undesirable. Loss of the fine dots or undercutting while the intaglio remains good is one indication of a reduced exposure latitude. Another indication is the filling in of the intaglio image when the exposure is increased to hold the fine dot relief. Optimally, a good exposure latitude might be defined as the ability of the plate to yield acceptable relief and intaglio detail over a wide range of exposure times.
Excessive scattering or irregular reflection of light from the support of the printing element is known as halation and can cause fogging or partial crosslinking of unexposed areas in the photosensitive layer. These "fogged" areas are difficult to dissolve during development, and can produce shallow reliefs that interfere with accurate printing.
In the prior art, actinic radiation absorbing or antihalation layers between the photosensitive layer and the support have been employed to reduce fogging. This layer absorbs light passing through it, whether directly or reflected from the support.
Conventional wisdom in the flexographic printing industry, however, teaches that the light absorbing materials cannot be used within the photosensitive composition of the photosensitive layer and must always be part of a separate layer in the flexographic plate. It was further believed that diffusible absorbers migrating from the antihalation layer into the photosensitive layer would interfere with crosslinking and produce an inferior plate. Absorbers in large amounts added directly to the photosensitive composition would similarly absorb the light essential for crosslinking as it passes through the layer, again producing an inferior plate. In addition, these absorbers could also reduce adhesion of the photosensitive layer with the adhesive used for adhering to a support.
One object of the invention is to produce a photosensitive composition where the speed of crosslinking is controlled and the exposure latitude is enhanced.
Another object of the invention is to produce a photosensitive composition and element for flexographic printing that is desirably soft, yet possesses flexibility, resilience and durability.